1. Field of the Invention
This invention relates in general to flow meters and, more particularly, to a vortex detector with enhanced sensitivity and signal processing for sensing and measuring vortex frequencies at very low flow rates.
2. Description of the Prior Art
Vortex shedding flow meters have been used for many years for a wide variety of applications and have proven to be quite popular because of their ability to measure the flow rates of a wide range of fluids accurately and reliably, including steam, liquids, and gases. A vortex shedding flow meter operates on the principle that a bluff body, when placed in a moving fluid, produces an alternating series of vortices, called a vortex street, at a frequency that is directly related to the velocity of the moving fluid. The amplitude of the each vortex is proportional to the square of the frequency of the vortex street. Some vortex shedding flow meters detect the frequency of the shed vortices, thus the flow rate, by having a vane positioned downstream from the bluff body. As the vortices in the vortex street pass over the vane, alternating lateral forces deflect the vane one way and then the other in much the same way that a flag furls in the wind in response to the vortices shed from the flag pole. The deflections of the vane can be detected and measured. The strengths of the vortices in the vortex street are related to the density of the fluid and its velocity. Therefore, high density, high velocity fluids produce strong vortices, while the vortices produced in low density, low velocity fluids are relatively weak.
One of the primary advantages of vortex shedding flow meters is that they have no moving parts, other than the flexure of the vane, bluff body, or other structure used as the transducer, and their inherent ruggedness makes them ideally suited for applications that involve extreme temperatures and pressures. However, one of the most serious disadvantages of vortex shedding flow meters is their inability to detect vortices in gases or other low density fluids very accurately as well as their inability to detect and measure vortices in fluids flowing at very low flow rates accurately. It has been very difficult, if not practically impossible, to detect in an accurate and dependable manner the very small vane deflections that result from the weak vortices produced in low speed flows of low density fluids, including liquids such as water.
Another disadvantage associated with currently available vortex shedding flow meters is that their signal to noise ratios are relatively low. Since transducers are typically used to detect the mechanical reaction of the vane to the passing vortices in the vortex street, they also pick up the other mechanical movements of the vane as well as vibrations and other noise in the fluid and in the pipe in which the flow meters are mounted, which can include the structural vibrations of pipe lines, low frequency acoustical noises penetrating the pipe wall, noises associated with flow fluctuations unrelated to the vortex street, and the like. The adverse effect of a low signal to noise ratio becomes particularly serious when trying to measure low speed flows of fluids, especially low density fluids, since the vortices themselves are quite weak. Therefore, the correspondingly weak signals produced by the vane deflection transducers may be lost or undetectable in the background noise.
One solution to the vortex detection problem associated with low density fluids has been to use ultrasound to detect the frequency of the vortices in the vortex street. Unfortunately, however, such ultrasonic vortex detection is not without its own drawbacks, including the errors introduced by bubbles and particles suspended in the fluid, as well as a general lack of ruggedness and durability, which makes them undesirable for use in high temperature, high pressure flow conditions.
The patent issued to Lew et al., U.S. Pat. No. 4,699,012, solves some of the shortcomings of the prior art vortex shedding flow meters by teaching the use of piezo-electric transducers to measure the deflection of the vane. Lew also achieves an improvement in the signal to noise ratio by mounting the vane on a thin diaphragm-like structure to increase the magnitude of the vane deflection, thus also increasing the magnitude of the output signal from the transducers. While Lew's vortex meter does achieve an improvement in signal to noise ratio over the prior art, additional improvements to signal to noise ratio would further enhance the usefulness of vortex shedding flow meters, particularly in the measurement of low velocity and low density fluids.